1) Field of the Invention
This invention relates to a novel mercapto compound and its preparation process, sulfur-containing urethane resins using the mercapto compound and lenses made of the resins.
The mercapto compound according to the present invention has a variety of utilities, for example, as a crosslinking agent, epoxy resin hardener, curing agent, polymerization regulator, synthetic resin raw material, antioxidant, metal complex former, biochemical reagent and lubricant additive.
2) Description of the Related Art
Plastic lenses are lightweight and fragmentation-resistant compared with inorganic lenses and, moreover, are dyeable, so that they are finding rapidly increasing utility as optical elements, such as eyeglass lenses and camera lenses, in recent years.
Resins widely employed these days for such purposes include the resin available by radical polymerization of diethylene glycol bis(allylcarbonate) (hereinafter called "D.A.C."). This resin has various advantages such as excellent impact resistance, lightweight, superb dyeability, and good workability such as good cutting and grinding machinability.
The refractive index of a D.A.C. lens, however, is 1.50 (n.sub.D =1.50) which is lower than the refractive index (n.sub.D =1.52) of an inorganic lens. To obtain optical characteristics comparable with those of a glass lens, it is necessary to increase the central thickness, peripheral thickness and curvature of the D.A.C. lens, unavoidably resulting in an overall large thickness. There is accordingly an outstanding desire for the development of a lens resin having a still higher refractive index. Known examples of lens resins which can provide a high refractive index include polyurethane resins available by reacting an isocyanate compound with a hydroxyl compound such as diethylene glycol (U.S. Pat. No. 4,443,588), a halogen-containing hydroxyl compound such as tetrabromobisphenol A (Japanese patent Laid-Open No. 164615/1983) or a hydroxyl compound having the diphenyl sulfide skeleton (Japanese patent Laid-Open No. 194401/1985).
Although lenses made of these known resins have a refractive index improved over lenses made of D.A.C., they are accompanied by the drawback(s) that they may be insufficient in refractive index and/or they may be poor in weatherability and/or high in specific gravity due to the use of a compound containing many halogen atoms or an aromatic ring in the molecule thereof with a view toward imparting an improved refractive index.
The present assignee has already filed an application for patent on 1,2-bis[(2-mercaptoethyl)thio]-3-mercaptopropane (U.S. Pat. No. 5,087,758). When this compound is polymerized with m-xylylene diisocyanate employed most widely for the production of a lens resin, the withstandable maximum temperature of the resulting resin is 98.degree. C. A lens made of this resin is therefore formed at temperature conditions (90.degree.-95.degree. C.) under which plastic lenses are usually dyed, leading to the desire for further improvements in heat resistance.
In addition, the color hue upon polymerization is important. The raw material of the plastic lens requires that its vapor pressure is not too high to permit ready distillation for its purification on an industrial scale. From this viewpoint too, it has been desired to develop a mercapto compound having a molecular weight of about 200.